It is practically preferred that lubricating oils employable as engine oils, hydraulic oils, etc. undergo least viscosity change over a wide temperature range from low temperatures to high temperatures. Viscosity index is employed as a measure of this property, and the greater the viscosity index is, the higher is the stability to temperature changes. It is known that viscosity index of an oil can be improved by adding to it a certain kind of polymer. As such polymers, for example, polymethacrylates (PMA) (Japanese Unexamined Patent Publication No. Hei 7-62372), olefin copolymers (OCP) (Japanese Patent Publication No. Sho 46-34508), hydrogenated styrene/diene copolymers (SDC) (Japanese Patent Publication No. Sho 48-39203) and polyisobutylenes (PIB) are employed. SDCs assuming polymeric forms of block copolymer (Japanese Unexamined Patent Publication No. Sho 49-47041) and star-shaped polymer (Japanese Unexamined Patent Publication No. Sho 52-96695) have been developed in addition to SDCs assuming a form of random copolymer.
Lubricating oils incorporated with these polymers exhibit characteristics of their own. More specifically, although PMAs have excellent viscosity index improving properties and also have pour point-lowering actions, they have poor thickening effects. In order to improve the thickening effects, the molecular weight of PMAs may be increased. However, PMAs having increased molecular weights come to have extremely low stability to shear forces to be caused by stirring etc. of lubricating oils. PIBs have high thickening effects but poor viscosity index improving properties. OCPs and SDCs have high thickening effects and have low viscosity at low temperatures, but their viscosity index improving properties are inferior to those of PMAs. Meanwhile, those PMAs which are copolymerized with polar monomers can impart to lubricating oils detergent dispersing performance of dispersing sludge in the lubricating oils easily compared with other improvers (Japanese Patent Publication No. Sho 51-20273 and Japanese Unexamined Patent Publication No. Hei 5-222389).
While multi-grade oils having excellent viscosity index improving performances are now generally used as lubricating oils, viscosity index improvers having higher performance are now in demand so as to comply with recent requirements including reduction of energy consumption. It can be contemplated to use a mixture of PMA and OCP or SDC as a composition which satisfies such requirements. However, these compounds have poor compatibility with each other, so that if a mere mixture of such compounds is added to a lubricating oil, the lubricating oil separates into two phases. Therefore, in order to avoid such separation, there are proposed graft copolymers consisting of two different kinds of polymers (Japanese Patent Publication Nos. Sho 59-40194, Sho 62-16997 and Hei 4-50328; Japanese Unexamined Patent Publication No. Hei 6-346078, etc.).
However, the processes for producing such graft copolymers each employ a conventional radical polymerization initiator to carry out graft copolymerization of a methacrylate monomer in the presence of an oil-soluble olefin copolymer. The graft copolymerization in this case takes place based on olefin copolymer chain radicals to be formed when the radical polymerization initiator extracts hydrogen atoms from the olefin copolymer. However, a homopolymer of the methacrylate monomer is also formed in a large amount when the graft copolymerization is carried out to bring about a low graft efficiency.
Accordingly, such graft copolymers involve a problem in that they have poor properties as viscosity index improvers including thickening effect, viscosity behavior, etc. Further, in those graft copolymers having extremely low graft efficiency values, lubricating oil compositions incorporated with them undergo separation. Accordingly, in order to comply with all conceivable severer requirements for lubricants in the future, development of high-performance viscosity index improvers is in demand.
The present invention was accomplished in view of such problems inherent in the prior art, and it is an objective of the invention to provide a viscosity index improver which has excellent viscosity index improving property, excellent thickening effect, excellent low-temperature fluidity and excellent shear stability and which is free from the fear of separation in a lubricating oil, as well as, a process for producing the same and a lubricating oil composition containing the same.